1. Field of the Invention
The invention relates to processes for producing raw material powders for growing single crystals of lithium potassium niobate or lithium potassium niobate-lithium potassium tantalate solid solution, and processes for producing the single crystals using said raw material powders.
2. Description of the Related Art
A single crystal of lithium potassium niobate and a single crystal of lithium potassium niobate-lithium potassium tantalate solid solution are remarked especially as single crystals for a blue light second harmonic generation (SHG) device for a semiconductor laser. The device can emit even ultraviolet light having wavelengths of 390 nm or so, thus the crystals can be suitable for wide applications such as for optical disk memory, medicine and photochemical fields, and various optical measurements by using such short-wavelength light. Since the above single crystals have a large electro-optic effect, they can be also applied to optical memory devices using the photo-refractive effect.
However, for an application of a second harmonic generation device, for example, even a small fluctuation in a composition of the single crystal may affect the wavelength of the second harmonic wave generated by the device. Therefore, the specification of the range of the composition required for the single crystals is severe, and the fluctuation in the composition should be suppressed in a narrow range. However, since the composition consists of as many as three or four components, growing a single crystal at a high rate is extremely difficult to achieve while controlling the proportions of the components to be constant.
NGK Insulators, Ltd. suggested a micro pulling-down method for growing the above single crystal with constant compositional proportions, for example, in JP-A-8-319191. In this method, a raw material comprising lithium potassium niobate is put into a platinum crucible and melted, and then the melt is pulled down gradually and continuously through a nozzle attached to the bottom of the crucible. An infrared irradiator and a detector are installed underneath the nozzle. Infrared rays are irradiated to the single crystal being pulled out through the nozzle, and the wavelength of the second harmonic wave oscillated from the single crystal is measured to control the composition of the single crystal constant.
Such a micro pulling-down method is useful for growing the above oxide single crystal having many components. However, for widespread uses of the above single crystal, increased industrial productivity is indispensable. For this purpose, the capacity of the crucible needs to be increased to grow as many single crystals as possible.
The inventors further examined this technique and found that, in case of mass production by enlarging the capacity of the crucible, the fluctuation in composition, development of cracks, inclusions and/or, in some cases, the formation of other nuclei, or crystalline deterioration accompanying anomalous growing rate, which hardly occurred in small-scale experiments, might be caused depending on the particle size of the raw material powder and the shape of the crucibles, which sometimes resulted in a yield loss.
An object of the invention is to prevent the fluctuation in the single crystal composition, the development of cracks or inclusions, and the crystalline deteriorations on growing a single crystal of lithium potassium niobate or a single crystal of lithium potassium niobate-lithium potassium tantalate solid solution.
A first aspect of the invention relates to a process for producing a raw material powder comprising lithium potassium niobate to grow a single crystal of lithium potassium niobate. The process comprises the steps of mixing raw starting materials comprising lithium carbonate powder, potassium carbonate powder and niobium pentoxide powder in a solvent, entirely dissolving the lithium carbonate powder and potassium carbonate powder into the solvent, then depositing lithium carbonate and potassium carbonate around the niobium pentoxide powder by spray-drying the mixture to obtain granulated powder, and thermally treating the granulated powder to produce the raw material powder.
According to the invention, a single crystal of lithium potassium niobate can also be grown by placing and melting the above raw material powder in a crucible, contacting a seed crystal against the melt, and pulling down the seed crystal from the crucible downwardly.
The invention is also related to a process for producing a raw material powder comprising lithium potassium niobate-lithium potassium tantalate solid solution to grow a single crystal of lithium potassium niobate-lithium potassium tantalate solid solution. The process comprises the steps of mixing raw starting materials comprising lithium carbonate powder, potassium carbonate powder, niobium pentoxide powder and tantalum pentoxide powder in a solvent, entirely dissolving the lithium carbonate powder and potassium carbonate powder into the solvent, then depositing lithium carbonate and potassium carbonate around said niobium pentoxide powder and said tantalum pentoxide powder by spray-drying the mixture to obtain granulated powder, and thermally treating the granulated powder to produce the raw material powder.
According to the invention, a single crystal of lithium potassium niobatelithium potassium tantalate solid solution can also be grown by placing and melting the above raw material powder in a crucible, contacting a seed crystal against the melt, and pulling down the seed crystal from the crucible downwardly.
In the first aspect of the invention, lithium carbonate and potassium carbonate depositing around the niobium pentoxide powder (and tantalum pentoxide powder, if necessary) are considered to be in fine powder forms.
The inventors found that the above fluctuation in the composition, cracks, inclusions and crystalline deteriorations were caused in accordance with the state change of the raw material powder, for example, when the capacity of the crucible was enlarged.
For example, when the raw material powder is supplied to the crucible, the raw material powder is melted and convected in the crucible, and then the melt is pulled out from the crucible. However, if the raw material powder has a heterogeneous portion, the composition becomes heterogeneous topically or temporary, which causes the fluctuation in the composition of the single crystal pulled out from the crucible. Development of cracks, inclusions or crystalline deteriorations may also occur topically depending on the fluctuation in the composition.
The existence of secondary particles in the potassium carbonate powder or the lithium carbonate powder as a raw starting material is considered as a cause of occurrence of the above heterogeneous portions in the raw material powder. Such starting material powders are hydroscopic, so that they tend to absorb moisture in air during the storage and aggregate to form the secondary coarse particles having particle diameters from 10 to 50 xcexcm. As the secondary coarse particles cannot sufficiently react with other components in the stage of heat treatment, they remain as heterogeneous portions in the raw material powder.
It is found that the raw material powder of lithium potassium niobate or lithium potassium niobate-lithium potassium tantalate solid solution produced by the heat treatment of the above granulated powder according to the invention can substantially eliminate the above heterogeneous portions to homogenize the composition of the single crystal pulled out from the crucible and to prevent cracks, inclusions and the crystalline deteriorations.
That is, the raw material powder is obtained in such a manner that among the starting powders, the potassium carbonate powder and the lithium carbonate powder are dissolved and then deposited again on the surface of the granulated powder by spray-drying and that the granulated powder is then subjected to a heat treatment to react potassiun, lithium and niobium (and tantalum, if necessary). It is found that the obtained raw material powder is extremely well suited for growing the single crystal of the above systems.
From further examination, the inventors found that, in the step of mixing the startomg materials, even when a part of the lithium carbonate powder was only partially dissolved, the above heterogeneous portions were also substantially eliminated to homogenize the composition of the single crystal pulled out from the crucible and to prevent cracks, inclusions and the crystalline deteriorations.
That is, when the lithium carbonate powder is not entirely dissolved into the solvent in the mixing step, the dissolved lithium carbonate deposits on the niobium pentoxide power (and tantalum pentoxide powder, if necessary) in the granulation step. On the other hand, the powder undissolved in the solvent during the mixing step remains in situ in the granulated powder and, even after the heat treatment, in the raw material powder.
Herein, the inventors found that the residual secondary particle of the lithium carbonate powder having particle diameters of 10 xcexcm or more in the raw material powder might cause the above fluctuation in the composition, cracks and inclusions. In other words, the inventors ascertained that, in this case, the above fluctuation in the composition, cracks and inclusions had their cause in the formation of the secondary particles by aggregating the lithium carbonate powder.
If such secondary particles are not substantially observed, at least at the stage of the raw material powder, the occurrences of the above fluctuation in the composition, cracks and inclusions can be prevented without dissolving the entire amount of lithium carbonate powder into the solvent.
Since potassium carbonate has a high solubility to a solvent, a small amount of the solvent can dissolve the entire amount of potassium carbonate. In contrast, since lithium carbonate has a lower solubility than that of potassium carbonate, a greater amount of the solvent is required to dissolve the entire amount of lithium carbonate. Therefore, if the throughput of the raw materials is high on dissolving the entire amount of lithium carbonate powder, a spray-drying operation will require more time and operational efficiency will be decreased. On the other hand, by dissolving a part of lithium carbonate powder, while leaving the rest undissolved in the solvent, the required amount of solvent is relatively decreased and a spray-drying operation can also be done within a shorter period of time.
At the stage of the raw material powder, the presence of secondary particles of the lithium carbonate powder having particle diameters of 10 xcexcm or more is inspected with an electron microscope. When ten view fields of at least 80 xcexcmxc3x97120 xcexcm are observed with an electron microscope (1000 magnification), observing no or little (preferably one or less) secondary particles of the lithium carbonate powder having the particle diameter of 10 xcexcm or more satisfies the above requirement.
To exclude secondary particles of the lithium carbonate powder having the particle diameters of 10 xcexcm or more at the stage of the starting material powder, for example, the lithium carbonate powder is sorted in the dry state at the stage of measuring and preparing the starting material powder. By doing so, when secondary coarse particles exist in the lithium carbonate powder, such secondary particles are excluded so that the secondary particles of the lithium carbonate powder having particle diameters of 10 xcexcm or more may not be included in the starting materials. Such a sorting method itself is commonly known.
Moreover, even when the secondary particles having particle diameters of 10 xcexcm or more comprising lithium carbonate powder exist in the starting materials, such particles can be crushed on mixing. More specifically, by increasing the hardness of balls or agitating media, enhancing the agitation rate or elongating the period of agitation time in a ball mill or a media agitating mill, a condition can be set to crush the secondary particles.
The invention can be favorably applied for producing not only single crystal fibers, but also plates comprising the single crystal.
When the potassium carbonate powder and the lithium carbonate powder are entirely dissolved, the average particle diameter of each of the potassium carbonate powder and lithium carbonate powder is not particularly limited. Although the average diameter of each of the niobium pentoxide powder and the tantalum pentoxide powder is not particularly limited, it is preferably 10 xcexcm or less from the view of acquiring the homogeneous raw material powder by a subsequent heat treatment, and preferably 0.1 xcexcm or more from the view of easier handling.
The proportions of the constituting elements are not particularly limited as far as the single crystal can be eventually grown. However, when a single crystal of lithium potassium niobate is produced, the proportions of the lithium carbonate powder, the potassium carbonate powder and the niobium pentoxide powder are preferably 17-27 mol %, 28-32 mol % and 43-53 mol %, respectively. Moreover, when a single crystal of lithium potassium niobate-lithium potassium tantalate solid solution is produced, the proportions of the lithium carbonate powder, the potassium carbonate powder, the niobium pentoxide powder and the tantalum pentoxide powder are preferably 17-27 mol %, 28-32 mol % 38.7-52.5 mol % and 0.2-5.3 mol %, respectively.
The single crystal of lithium potassium niobate and that of lithium potassium niobate-lithium potassium tantalate solid solution may be replaced by other elements other than K, Li, Nb, Ta and O within a range of taking a tungsten bronze structure consisting of K, Li, Nb and, if necessary, Ta and O. For example, Na and Rb can substitute K and Li. In this case, the substitution rate is preferably 10 atomic % or less, when the proportion of potassium or lithium is taken as 100 atomic %. A laser-generating doping substance comprising such as Cr or a rare earth series elements of Er or Nd can also be added.
According to an embodiment of the invention, the lithium carbonate powder and the potassium carbonate powder are entirely dissolved into a solvent. In this case, whether each powder can be entirely dissolved depends on the solubility of that powder to the solvent and the volume of the solvent. Therefore, a sufficient amount of the solvent needs to be used to dissolve each powder, and adequate agitation and mixing are preferable.
The solvent is not particularly limited, but water and acids are especially preferable in that the solubilities of the potassium carbonate powder and the lithium carbonate powder thereto are high. Lithium carbonate powder has high solubilities to acids.
Also, by adding an organic binder into the solvent, potassium carbonate powder and lithium carbonate powder precipitated in water can easily attach to the surface of niobium pentoxide or tantalum pentoxide in the subsequent spray-drying. Such binders that are usable in an alkaline solution in which potassium carbonate and lithium carbonate are dissolved, acrylic binder, for example, are preferably used as the organic binder. The weight ratio of the organic binder per 100 parts by weight of the solvent is preferably 0.1-0.3 wt. %.
Such conditions as temperature, period of time and atmosphere for the heat treatment of the granulated powder are not particularly limited as far as each of the components of the granulated powder adequately react to produce the respective compounds. However, in general, the temperature is preferably 800-1,000xc2x0 C. and the holding time is at least 2 hours.
FIG. 1 is a schematic sectional view of a manufacturing apparatus according to one example of the present invention for the growth of single crystals.
A crucible 7 is placed in a furnace housing. An upper furnace unit 1 is arranged to surround the crucible 7 and an upper space 5 thereof, and has a heater 2 buried therein. A nozzle 13 extends downwardly from a bottom part of the crucible 7. An opening 13a is formed at a lower end of the nozzle 13. A lower firnace unit 3 is arranged to surround the nozzle 13 and a surrounding space 6 thereof, and has a heater 4 buried therein. Such a configuration of the heating furnace itself may be obviously varied in various ways. The crucible 7 and the nozzle 13 are both formed of a corrosion-resistant conductive material.
One electrode of a power source 10A is connected to a point A of the crucible 7 with an electric cable 9, and the other electrode of the power source 10A is connected to a lower bent B of the crucible 7. One electrode of a power source 10B is connected to a point C of the nozzle 13 with the electric cable 9, and the other electrode of the power source 10B is connected to a lower end D of the nozzle 13. These current-carrying systems are isolated from each other and configured to control their voltage independently.
An after-heater 12 is further located in the space 6 to surround the nozzle 13 with a distance. An intake tube 11 extends upward in the crucible 7 and an intake opening 22 is provided at the upper end of the intake tube 11. The intake opening 22 protrudes from a bottom portion of a melt.
The upper furnace unit 1, the lower furnace unit 3 and the after-heater 12 are allowed to heat for setting an appropriate temperature distribution for each of the space 5 and space 6. Then a raw material of the melt is supplied into the crucible 7 and the electricity is supplied to the crucible 7 and the nozzle 13 for heating. In this condition, at a single crystal-growing part 23 located at the bottom end of the nozzle 13, the melt 8 slightly protrudes from the opening 13a and a relatively flat surface is formed there. A reference numeral 30 denotes the liquid level of the melt.
The gravity applied to the melt 8 in the nozzle 13 is greatly reduced due to the melt contacting against the inner surface of the nozzle 13. Especially, a uniform solid phase-liquid phase interface can be formed by setting the internal diameter of the nozzle 13 to 0.5 mm or less.
In this condition, a seed crystal is moved upward and the end face of the seed crystal is contacted with the surface of the melt. Then, the seed crystal is pulled downward. Hereupon, a uniform solid phase-liquid phase interface (meniscus) is formed between the upper end of the seed crystal and the melt being pulled out from the nozzle.
As a result, a single crystal fiber 14 is continuously formed on the upper side of the seed crystal and pulled downward. According to this embodiment, a roller 28, which is a drive unit, carries the single crystal fiber 14.
When the single crystal is continuously pulled out downwardly, a laser beam having a wavelength of near 2xcex0 is radiated from a laser source 27 to the single crystal 14 as shown by an arrow R in FIG. 1 indicating, and a output light S from the single crystal having a wavelength of near a second harmonic wavexcex0 is received by a light-receiving unit 26 through a long wavelength cut filter 41 to detect the intensity thereof A signal from the light-receiving unit 26 is transmitted to a control unit 33 through a signal line 25, wherein the signal is processed. When the measured intensity of the output light deviates from a predetermined value, the control unit transmits a certain signal to a raw material supplying apparatus 24 as a feedback. In this case, a raw material powder having a slightly different composition from the raw material powder first placed into the crucible may be further added into the crucible.
For more accurate control, a part of the long wavelength light of near 2xcex0 is measured by a combination of a reflecting mirror 29 and the light-receiving unit 26, and the resulting signal is transmitted to the control apparatus 33 through the signal line 25.
A single crystal plate can be formed by changing the shape of the nozzle 13.
Hereafter, further detailed experimental results are described.